1. Field of the Invention
The invention relates to endoscopic apparatus for searching for and detecting defects in parts that are substantially inaccessible, being located behind a wall.
2. Description of the Background
It is already known to inspect internal components of equipment or a machine by means of an endoscope, i.e. an elongate optical instrument of small diameter that is passed through a small orifice in a wall in order to examine the appearance of components that are located on the other side of the wall. This makes it possible in particular to inspect the blades of a turbomachine such as an airplane turbojet or turboprop, without dismantling the turbomachine.
Nevertheless, the ability to detect defects is limited by the resolution of the optical system of the endoscope, by the illumination, by the orientation of the endoscope relative to the surface under observation, and by the nature of the defects, amongst other things.
It is also known to seek defects on machine components by using a penetration test technique, which consists in depositing organic substances on the surface under examination of a component and in observing the results under ultraviolet illumination. It is thus possible to detect surface discontinuities such as cracks or fissures that are of very small dimensions, and that are invisible under normal illumination, by depositing a fluorescent dye on the component, which dye penetrates into the surface discontinuities, and then by washing and baking or drying the corresponding surface of the component, and subsequently depositing a developer to reveal the fluorescent dye that has infiltrated into the surface discontinuities. The component is then illuminated in ultraviolet radiation which excites the fluorescent dye, which responds by emitting radiation at some other wavelength, for example of a green color in the visible spectrum.
This penetration test technique generally requires the machine under examination to be disassembled more or less completely.
Proposals have already been made, in document U.S. Pat. No. 4,273,110, to combine the techniques of endoscopy and of penetration testing in a single instrument which comprises, in an elongate cylindrical sheath of small diameter, a quartz tube for transmitting ultraviolet light and visible light towards an end of the instrument that is close to the component under examination, a light pipe containing lenses to observe an illuminated zone of the component, and three pipes for feeding and spraying penetration test substances onto the component under examination. An optical filter that stops visible light is removably mounted at the other end of the instrument between the quartz tube and a light source that emits in the visible and in the ultraviolet, so as to enable the component to be illuminated alternately in visible light and in ultraviolet light. The outside diameter of the sheath of the instrument is small enough to enable it to pass through an endoscope orifice having a diameter of about 9 millimeters (mm).
That prior art instrument presents several drawbacks:
The illumination of the component in ultraviolet light takes place via an axial outlet in order to avoid using reflecting mirrors which are excessively penalizing 30 in terms of size and light absorption. The outlets of the spray pipes are necessarily axial, as are the outlets for illumination and for the observation means, thereby greatly restricting the use of the instrument. In addition, the section of the observation light pipe is very small, since said pipe must leave sufficient space inside the sheath to enable the quartz tube and the pipes for feeding and spraying penetration test substances to be installed, such that when the instrument is used with ultraviolet illumination, it can pick up only a small fraction of the light emitted by the penetration test substances, and therefore does not enable observation to be performed reliably.